A pre-stressing device, and a method for reinforcing a structural member

ABSTRACT

A pre-stressing device for reinforcing and/or repair a structural member by means of a pre-stressed cover plate is provided. The device comprises a stress member being connected to an attachment structure or at least temporarily securing said device to the cover plate, wherein the stress member is configured such that when a force is applied to the stress member, different portions of the stress member will immediately be subjected to different magnitudes of force.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Swedish Patent Application No. 1351579-6 filed Dec. 23, 2013 and PCT/EP2014/079140 filed Dec. 23, 2014, which are hereby incorporated by reference in their entirety.

TECHNICAL FIELD

The present disclosure relates to a pre-stressing device and a method for attaching a pre-stressed cover plate, such as a fiber reinforced polymer (FRP) laminate or a steel reinforced polymer (SRP) laminate or a steel reinforced grout (SRG) composite or a steel plate, to a structural member, such as a part of a bridge, building, vehicle or any other structural member.

BACKGROUND

It is known that the benefits of bonding cover plates to structural members may be increased by inducing pre-stressing in the cover plate when bonding it to a structural member. A cover plate is namely pre-stressed and bonded to a structural member using a curable bonding material such as an adhesive while maintaining the stressing force. The stressing force is released when the bonding material has hardened or cured. Pre-stressing the cover plates before bonding them to structural members has several advantages. Although the bonding of cover plates could be applied to a variety of materials forming the structural member to be reinforced, particular advantages are obtainable when it comes to reinforcing concrete structural members. When bonding a pre-stressed cover plate to a concrete structure these advantages include: a reduction in deformations due to live loads and thus performance enhancement in the serviceability limit state, an increase in the load carrying capacity of the concrete member, crack width reduction on the tensile part of the structure and consequently an increase in durability, the provision of a negative moment against dead loads and more capacity for live loads, and a compensation for the lost pre-stress in a pre-stressed concrete structure which may e.g. be caused by corrosion or damage of tendons.

When bonding an FRP laminate to a steel structure the advantages include the enhancement of the fatigue strength of the steel structure and the prevention of fatigue crack formation or propagation in the steel structure.

In general, pre-stressing results in a reduction in the needed cross sectional area of the cover plate and thus saving of the material

A problem when using bonded pre-stressed cover plates is that high shear stresses may build up at the ends of the cover plate in the adhesive layer that bonds the cover plate to the structural member. These shear stresses are normally several times higher than the adhesion strength of conventional adhesives, such as epoxy or the strength of the substrate material, e.g. concrete. The shear stresses may give rise to detachment of the cover plate from the structural member, whereby the separation may be initiated at the ends of the cover plate and thereafter propagate inwards from the cover plate end.

Mechanical anchors are usually used to solve the problem of high shear stresses at the cover plate ends. However, there are several problems associated with using a mechanical anchoring system. Mechanical anchors are in many cases rather complicated, time-consuming and costly to manufacture, install and inspect. Further, they often need to be manufactured with very close dimensional tolerances for the specific structural member to be strengthened. The structural member on which they are mounted often needs to be modified, e.g. such that a part of the structural member may need to be cut out and removed and bolts may have to be drilled into the structural member and fixed in place using adhesive or mortar bonding. The mechanical anchors may be susceptible to moisture and dust accumulation which may result in the corrosion of the anchoring system. Furthermore, galvanic corrosion may take place when metal anchors are used on a structure comprising a dissimilar metal.

U.S. Pat. No. 6,464,811 discloses a method of reinforcing a construction part with lamellar, fiber-reinforced plastic strips. The lamellar strips are pre-tensed with a tensioning device, treated with adhesive in a pre-tensed state and then moved to the construction part to be treated together with a tension device. The tension device is provisionally fixed to the construction part with displaceable fixing devices. Thereafter, the lamellar strips are pressed against the construction by means of an air bag or air hose until the adhesive has hardened. The strips may be pre-stressed by different amounts by pre-tensioning a first part of the strip using a first tension and adhering that first part of the strip to the construction part, and then, once the adhesive has cured, pre-tensioning a second part of the strip using a second tension and then adhering that second part of the strip to the construction part. This method is however quite time consuming and complex, especially if long strip lengths are used, and, if an existing structure, such as a bridge, is being reinforced; it could be out of service for a considerable period of time.

U.S. Pat. No. 2011/0000606 discloses a method for applying a reinforced composite material to a structural member. The surface of the composite laminate is brought to the structural member and is pre-stressed with a device, while there is a curable adhesive on either the composite laminate or the structural member. This patent discloses a device comprising a series of tabs bonded to the composite laminate and a mold with recesses, attached to the structural member, used to stop the tabs. The recesses in the mold could be designed in a way that they give a desirable pre-stressing force profile to the composite laminate. A variable pre-stressing force could be induced in the fiber reinforced material by tensioning the composite laminate to full pre-stressing force and then releasing the laminate. The tabs will then come into contact with the mold in a successive manner and give a variable force profile in the composite laminate. After full curing of the adhesive, the mold is detached from the structure.

Although this kind of device is advantageous for several reasons, the device used in this method should have very small tolerances between the tabs and the mold in order to maintain the exact forces at each step, which is somewhat difficult to achieve in practice. Small errors in manufacturing of the tabs and the mold can affect the order in which the tabs come into contact with the mold, and changes the force distribution in the composite laminate. Therefore, an improvement in the design of this device, which could eliminate the need of small tolerances, could help to provide a more accurate and reliable force distribution in the composite laminate.

SUMMARY

An object of the present disclosure is to provide a device and a method for applying a pre-stressed cover plate, such as a fiber reinforced polymer (FRP) laminate or a steel reinforced polymer (SRP) or a steel reinforced grout (SRG) composite (i.e. a composite comprising steel cords formed from interwoven steel wires embedded within a polymer resin or cementations grout matrix) or a steel plate, to a structural member, such as at least part of a bridge (such as the span, a column, tendon, girder or hanger), a building (such as a wall, pillar, floor or roof), a vehicle or any other monolithic or polylithic structure for any reason such as strengthening and repair of the said member which eliminates the need for mechanical anchors at the ends of the pre-stressed cover plate after releasing the pre-stressing force.

An idea of the present disclosure is to provide a device and method which overcome the shortcomings of the existing devices and technologies. In this disclosure, the concept of successive contact of mechanical parts to provide a desirable force profile in the cover plate has been abandoned and the desirable force transfer from the source of the pre-stressing force to the cover plate is done immediately from the beginning of the procedure assuring a smooth and continuous force transfer to the cover plate.

According to a first aspect a pre-stressing device for reinforcing and/or repair a structural member by means of a pre-stressed cover plate is provided. The device comprises a stress member being connected to an attachment structure for at least temporarily securing said device to the cover plate, wherein the stress member is configured such that when a force is applied to the stress member, different portions of the stress member will immediately be subjected to different magnitudes of force. Since the stress is then distributed over the stress member in a predetermined manner being defined by the configuration of the stress member, the cover plate will be subject to same distribution.

The device may further comprise a pressure source for generating a pre-stressing force to the device by applying stress to the stress member. The pressure source may comprise a hydraulically or mechanically operated piston-cylinder unit, a screw link actuator, or a screw.

The stress member may comprise a body having tapered shape, or the stress member may comprise a series of tabs.

Each tab may be connected to an adjacent tab by means of at least one spring, and wherein the axial stiffness of said springs is decreasing along the length of the device.

Each tab may further be connected to a pressure source used for applying a pre-stressing force to the device by means of a spring.

In some embodiments, each one of said tabs is connected to an associated pressure source.

The device may be attached to the cover plate via an intermediate medium, such as a strip of fiber reinforced polymer, a wooden strip, or a plastic strip.

According to a second aspect, a method for reinforcing a structural member by applying a pre-stress to a cover plate being attached to the structural member is provided. The method comprises the steps of at least temporarily securing a stress member to the cover plate by means of an attachment structure, and applying force to the stress member such that when the force is generated, different portions of the stress member will immediately be subjected to different magnitudes of force.

According to a further aspect, a pre-stressing device for reinforcing and/or repair a structural member by means of a pre-stressed cover plate is provided. The device comprises a stress member being connected to an attachment structure for at least temporarily securing said device to the cover plate, wherein the stress member has a varying axial stiffness along a longitudinal direction such that when a force is applied to the stress member, different portions of the cover plate will immediately be subjected to different magnitudes of stress.

According to a further aspect, a pre-stressing device for reinforcing and/or repair a structural member by means of a pre-stressed cover plate is provided. The device comprises a stress member being connected to an attachment structure for at least temporarily securing said device to the cover plate, wherein the stress member comprises a body having a tapered shape, whereby the stress member has a varying axial stiffness along a longitudinal direction such that when a force is applied to the stress member, different portions of the cover plate will immediately be subjected to different magnitudes of stress.

According to a further aspect, a pre-stressing device for reinforcing and/or repair a structural member by means of a pre-stressed cover plate is provided. The device comprises a stress member being connected to an attachment structure for at least temporarily securing said device to the cover plate, wherein the stress member comprises a series of tabs, each one of said tabs being connected to an associated pressure source such that when different forces are applied to the individual tabs, different portions of the cover plate will immediately be subjected to different magnitudes of stress.

It should be noted that the expression “curable adhesive” is intended to include any type of material with any type of composition, capable of bonding two surfaces to each other such as any types of glue, epoxy resins and any types of concrete.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will hereinafter be further explained by means of non-limiting examples with reference to the appended schematic figures where;

FIG. 1 is a top view of a device according to an embodiment;

FIG. 2a is a side view of a structural member to which a cover plate is being applied using a device according to an embodiment;

FIG. 2b-c show the pre-stressing force distribution and force transfer to the cover plate when using the device shown in FIG. 2 a;

FIG. 3a is a side view of a structural member to which a cover plate is being applied using a device according to a further embodiment;

FIG. 3b-c show the pre-stressing force distribution and force transfer to the cover plate when using the device shown in FIG. 3 a;

FIG. 4a is a side view of a structural member to which a cover plate is being applied using a device according to a yet further embodiment;

FIG. 4b shows the pre-stressing force distribution and force transfer to the cover plate when using the device shown in FIG. 4 a;

FIG. 5a is a side view of a structural member to which a cover plate is being applied using a device according to a further embodiment; and

FIG. 5b-c show the pre-stressing force distribution and force transfer to the cover plate when using the device shown in FIG. 5 a.

DETAILED DESCRIPTION OF EMBODIMENTS

Starting with FIG. 1, two devices 100 according to an embodiment is shown. The devices 100 are used for applying a pre-stressed cover plate 112 to a structural member 10 by means of a curable adhesive 114 (see e.g. FIG. 2a ). As can be seen in FIG. 1 the cover plate 112 extends in a longitudinal direction, and the devices 100 are arranged at a respective end of the cover plate 112.

During operation, a device 100 is at least temporarily attached to the cover plate 112 by means of an attachment structure 130, such as bolts or similar. The cover plate 112 is applied to the structural member 10 using e.g. a layer of curable adhesive 114. The device 100 is activated by applying a pre-stressing force at one end of the device, which force will immediately be transferred to the cover plate 112 in a manner such that the cover plate 112 is exposed to a pre-stress gradient. When the adhesive is cured the device 100 may be removed from the cover plate 112 which at this point is securely attached to the structural member 10.

The device 100 is preferably used for applying a cover plate 112, such as a fiber reinforced polymer (FRP) laminate or a steel reinforced polymer (SRP) laminate or a steel reinforced grout (SRG) composite or a steel plate, to a structural member 10, such as a part of a bridge, building, vehicle or any other structural member.

FIG. 2a shows an embodiment of the device 100. A structural member 10, e.g. in the form of a beam constituting part of the span of a bridge is subject to reinforcement. A cover plate 112 in the form of a strip, such as a pre-cured carbon fiber reinforced polymer (CFRP) laminate, has been applied to the structural member 10 by coating a surface of the structural member 10 with a continuous or discontinuous layer 114 of curable adhesive and the cover plate 112 is pressed against the adhesive-coated surface of the structural member 10.

At least one pre-stressing device 100 is connected to the cover plate 112 and comprises a stress member in the form of a series of tabs 118 connected to each other by means of springs 120 a-n. Each tab 118 is attached to the cover plate 112 (for example by means of bonding or mechanical joining 130) at a certain interval along the cover plate 112. A pre-stressing force, P_(max) is applied to the tab 118 a being attached to the cover plate 112 at the end of the cover plate 112. The tabs 118 are arranged along a treatment length, L_(n), and being connected to each other by means of a series of springs 120 a-n. The exact degree of pre-stressing force may be measured with strain gauges positioned on the cover plate 112, or by means of an integral force measuring device housed in the pre-stressing device 100 or in the source of pre-stressing force.

Preferably, the pre-stressing force is gradually applied from zero to P_(max) to the first tab, 118 a. A series of springs 120 a-n with predetermined axial stiffness K_(x) will transfer the force from the first tab 118 a to adjacent tabs 118 b-n in a way that the applied force on each tab 118 is substantially equal. Constitution of the stress member, i.e. the tabs 118 and the springs 120 a-n is so that the distribution of the pre-stressing force P_(max) among the tabs 118 will in this case result in an axial force in the cover plate 112 which is zero at the end of the cover plate 112 and increasing to P_(max) at the end of the treatment length L_(n). The concept of the force distribution, at one end of the cover plate 112 is shown in FIG. 2b and the axial force distribution in the cover plate is illustrated in FIG. 2c . It should be realized that the exact number of tabs 118 could be varied in accordance with the specific application for which the device 100 is used. Hence the number of tabs 118 is not the same for FIG. 2a and FIG. 2b , although the principle of this particular embodiment is applicable for any number of tabs 118.

The pre-stressing device 100 thus comprises a series of tabs 118, attached to the cover plate 112 along a treatment length, L_(n), at the end of the cover plate 112 and a series of springs 120 a-n with predetermined axial stiffness K_(x) connecting each tab 118 to the neighboring tabs 118. The tabs 118 and the springs 120 form a stress member. The pre-stressing force, P_(max) is applied to the first tab 118 a. The springs 120 will transfer the force from the first tab 118 a to other tabs in a way that the transferred force to each tab 118 is equal and has the same share from the pre-stressing force P_(max).

Hence the stress member 118, 120 is connected to the attachment structure 130 for at least temporarily securing the device 100 to the cover plate 112. The stress member 118, 120 has a varying axial stiffness due to springs 120 along a longitudinal direction such that when a force is applied to the stress member 118, 120 different portions of the cover plate 112 will immediately be subjected to different magnitudes of stress.

FIG. 3a shows a device 100′ according to a further embodiment. The figure shows a structural member 10 to which a cover plate 112 is being applied. A cover plate 112 in the form of a strip, such as a pre-cured CFRP laminate, has been applied to the structural member by coating a surface of the structural member 10 with a continuous or discontinuous layer of curable adhesive 114 and pressing the cover plate 112 against the adhesive-coated surface of the structural member 10.

At least one pre-stressing device 100′ is connected to the cover plate 112 and comprises a stress member in the form of a series of tabs 118′ and springs 120′a-n. The tabs 118′ are attached to the cover plate 112 (for example by means of bonding or mechanical joining) at a certain interval along the cover plate 112. A series of springs 120′a-n with predetermined axial stiffness K_(x) forms means for generating force to the stress member and connect each tab 118′ to the source of the pre-stressing force. These springs 120′a-n will equally distribute and transfer the pre-stressing force to the tabs 118′.

The pre-stressing force, P_(max), is applied to the springs 120′a-n and then transferred to the tabs 118′ with an equal portion of the pre-stressing force P_(max). The exact degree of pre-stressing force may be measured with strain gauges positioned on the cover plate 112, or by means of an integral force measuring device housed in the pre-stressing device 100′ or in the source used for generating the pre-stressing force.

The pre-stressing force is then gradually applied from zero to P_(max) to all the springs, 120′a-n at the same time. The predetermined stiffness of the springs 120′ will assure the same share of the pre-stressing force, P_(max), to each tab 118′. The distribution of the P_(max) in such a manner among the tabs 118′, will result in an axial force in the cover plate 112 which is zero at the end of the cover plate 112 and increasing to P_(max) at the end of the treatment length L_(n). FIGS. 3b-c illustrate the concept of the force distribution in the cover plate 112 according to this embodiment.

The pre-stressing device 100′ thus comprises a series of tabs 118′, attached to the cover plate 112 along a treatment length, L_(n), at the end of the cover plate 112 and a series of springs 120′a-n with predetermined axial stiffness K_(x) connecting each tab 118′ to the source 140 of the pre-stressing force. The tabs 118′ and the springs 120′ form a stress member. The pre-stressing force, P_(max) would be applied to all springs simultaneously. The springs 120′ would transfer the force from the source of the pre-stressing force to tabs 118′ in a way that the transferred force to each tab 118′ is equal and has the same share from the pre-stressing force P_(max).

FIG. 4a shows a device 100″ according to a yet further embodiment. The figure shows a structural member 10 to which a cover plate 112 is being applied. A cover plate 112 in the form of a strip, such as a pre-cured CFRP laminate, has been applied to the structural member by coating a surface of the structural member 10 with a continuous or discontinuous layer of curable adhesive 114 and pressing the cover plate 112 against the adhesive-coated surface.

A pre-stressing force is then applied, from zero to P_(max), to the pre-stressing device 100″ comprising a stress member in the form of a monolithic body 128 (made of for example metal or fiber reinforced polymer), with a certain predetermined shape, attached to the cover plate 112 (for example by means of bonding or mechanical joining being similar to the bolts 130 shown in FIG. 1) along a treatment length, L_(n), at the end of the cover plate 112. The exact degree of pre-stressing force may be measured with strain gauges positioned on the cover plate 112, or by means of an integral force measuring device housed in the pre-stressing device 100″ or at the source used to generate the pre-stressing force. The pre-stressing force is then gradually applied from zero to P_(max) to the pre-stressing device 100″. The shape of the stress member 128 will ensure a gradual and uniform force transfer from the pre-stressing device 100″ to the cover plate 112. Hence, the monolithic body 128 forms a stress member as well as means for generating stress. This distribution will thus result in an axial force in the cover plate 112 which is zero at the end of the cover plate 112 and increasing to P_(max) at the end of the treatment length L_(n). FIG. 4b illustrates the concept of the force distribution in the cover plate according to this embodiment.

The pre-stressing device 100″ thus comprises a monolithic body 128, attached to the cover plate 112 along a treatment length, L_(n), at the end of the cover plate 112. The monolithic body 128 forms a stress member. The monolithic body 128 has a predetermined shape and is connected to the source of the pre-stressing force at its end. The monolithic body 128 will transfer the force from the source 140 of the pre-stressing force to the cover plate 112 in a way that the force transfer has a uniform rate.

The stress member 128 is connected to the attachment structure 130 for at least temporarily securing said device 100″ to the cover plate 112, wherein the stress member 128 comprises a body having a tapered shape. The stress member 128 has a varying axial stiffness along a longitudinal direction such that when a force is applied to the stress member 128, different portions of the cover plate 112 will immediately be subjected to different magnitudes of stress.

FIG. 5a shows the device according to another embodiment. The figure shows a structural member 10 to which a cover plate 112 is being applied. A cover plate 112 in the form of a strip, such as a pre-cured CFRP laminate, has been applied to the structural member by coating a surface of the structural member 10 with a continuous or discontinuous layer of curable adhesive 114 and by pressing the cover plate 112 against the adhesive-coated surface.

At least one pre-stressing device 100′″ is connected to the cover plate 112 and comprises a series of tabs 118′″, wherein pre-stressing forces are applied simultaneously, gradually increasing from zero to ΔP, to the series of tabs 118′″. The tabs 118′″ are attached to the cover plate 112 (for example by means of bonding or mechanical joining 130) along a treatment length, L_(n), at the end of the cover plate 112. Hence, the tabs 118′″ form a stress member. The exact degree of pre-stressing force may be measured with strain gauges positioned on the cover plate 112, or by means of an integral force measuring device housed in the pre-stressing device 100′″ or at the source used for generating the pre-stressing force. Each pre-stressing force could be applied by means of for example a centrally controlled hydraulic jack or a mechanically steered system to each tab 118′″. The total amount of pre-stressing force, applied on a certain number of tabs 118′″ would be equal to P_(max). Applying the pre-stressing force in such a manner among the tabs 118′″ will result in an axial force in the cover plate 112 which is zero at the end of the cover plate 112 and increasing to P_(max) at the end of the treatment length L_(n). FIGS. 5b and 5c illustrate the concept of the force distribution in the cover plate 112 according to this embodiment.

The stress member 118′″ is connected to the attachment structure 130 for at least temporarily securing said device 100′″ to the cover plate 112, wherein the stress member 118′″ comprises a series of tabs 118′″, each one of said tabs 118′″ being connected to an associated pressure source 140 such that when different forces are applied to the individual tabs 118′″, different portions of the cover plate 112 will immediately be subjected to different magnitudes of stress.

According to the above description, the advantages of the device 100, 100′, 100″, and 100′″ may be realized by a method comprising the steps of applying a curable adhesive, such as an epoxy resin or any other suitable curable adhesive, to a surface of the structural member and/or a surface of the cover plate, bringing the surfaces into contact and applying a pre-stressing force P, from zero to P_(max), to the cover plate material via the device. The pre-stressing force is transferred to the cover plate in a number of steps or in a continuous manner.

In general the pre-stressing force may be applied using a pressure source 140 in the form of a hydraulically or mechanically operated piston-cylinder unit or by means of a screw link actuator or simply by means of a screw. The pre-stressing force, P_(max), will be transferred to the cover plate along the treatment length, L_(n). The method assures a gradient distribution of the P_(max), in discrete steps or a continuous manner along L_(n) to the cover plate.

The treatment length, L_(n) and the number of steps, n, through which the pre-stressing force, P_(max), to be applied to the cover plate, could vary and be designed in a way that it results to a desired distribution of stresses in the bond line between the cover plate and the structural member.

According to an embodiment the method comprises the step of inducing the pre-stressing force, from zero to P_(max), to which a treatment length L_(n) of the cover plate is subjected in a step-wise or continuous manner so that the cover plate along the treatment length, L_(n), will comprise a plurality of length sections each having a different pre-stressed state when the adhesive has cured.

According to an embodiment, the treatment length L_(n) is a length at an end of the cover plate, i.e. the treatment length L_(n) continues to the very end of a cover plate or stops just short of the end of the cover plate.

According to an embodiment the method comprises a step of connecting each neighboring tabs to each other by means of a series of springs. The first tab will be connected to the source of the pre-stressing force such as a hydraulic jack by means of a metal rod.

According to another embodiment the method comprises a step of connecting each tab to the source of the pre-stressing force via a respective spring.

According to a further embodiment the method comprises a step of continuously connecting a monolithic body with a specific shape to the cover plate. The body of the device is connected to the source of the pre-stressing force such as a hydraulic jack by means of a metal rod.

According to a further embodiment the method comprises a step of connecting a number of tabs to a number of pre-stressing force sources, such as hydraulic jacks. The jacks could be mounted on the device, the structure or a third party body, applying the same amount of force on each tab.

The above described device and method allows a pre-stressed cover plate 112 having a non-uniform axial pre-stressing force profile starting from zero at the end of the cover plate 112 increasing to P_(max) at the end of the treatment length along the cover plate 112. Such a pre-stressing force profile allows for using pre-stressed cover plates without having to use permanent mechanical anchors and thus avoiding the above-mentioned problems of prior art associated with permanent mechanical anchors. The pre-stressing process is simple, reliable and cost-effective and takes a short time, which limits disruptions and delays during the process of bonding the cover plate 112 such as disruptions and delays in the traffic flow over a heavily trafficked bridge for example, which can otherwise present a major problem when using conventional methods.

Very high pre-stressing forces (up to 1500 MPa) can be applied to the cover plate 112 without concentrating interfacial stresses along the adhesive layer 114 between the structural member 10 and the cover plate 112 at the ends of the reinforced composite material providing that a suitable treatment length, L_(n), and number of steps, n, has been chosen.

Finite element simulations of the adhesive bond line when the device is used, has confirmed that the magnitude of critical shear and peeling stresses at the ends of a pre-stressed cover plate material 112 can be reduced by a factor of hundred as compared to conventional methods in which a cover plate material 112 is adhered to a structural member 10 in uniformly pre-stressed state. Shear and peeling stresses at the ends of a pre-stressed cover plate material 112 may in fact be eliminated all together by leaving part of the laminate at the end stress-free.

It should be noted that the expression “cover plate” is intended to include any type of material with any type of composition, such as a sheet- or strip-like structure of any shape, size and thickness or a cable-like structure of any cross-sectional shape and comprising any type of material. Examples of cover plates include fiber reinforced polymer (FRP) laminates, steel plates and steel cables.

Further, the device 100, 100′, 100″, 100′″ could be attached to the cover plate 112 via an intermediate medium 112 a, as is shown in FIG. 2a . The intermediate medium could be mechanically connected to the cover plate 112, for example by means of fasteners or be adhesively bonded to the cover plate 112. The intermediate medium could in some embodiments be mechanically connected to the pre-stressing device, for example by means of fasteners or be adhesively bonded to the pre-stressing device. In a yet further embodiment, the intermediate medium could be built into or merged in the cover plate. In a preferred embodiment, the intermediate medium is arranged between the stress member and the cover plate 112.

The springs 120, 120′ could e.g. be made of metal or fiber reinforced polymer such as CFRP, or plastic for example with any cross sectional shape such as circular or rectangular for example. 

1. A pre-stressing device for reinforcing or repairing a structural member using a pre-stressed cover plate, comprising a stress member being connected to an attachment structure for at least temporarily securing said device to the cover plate, wherein the stress member is configured such that when a force is applied to the stress member, different portions of the stress member will immediately be subjected to different magnitudes of force.
 2. The device according to claim 1, further comprising a pressure source for generating a pre-stressing force to the device by applying stress to the stress member.
 3. The device according to claim 2, wherein said pressure source comprises at least one of a hydraulically or mechanically operated piston-cylinder unit, a screw link actuator, and a screw.
 4. The device according to claim 1, wherein the stress member comprises a body having a tapered shape.
 5. The device according to claims 1, wherein the stress member comprises a series of tabs.
 6. The device according to claim 5, wherein each tab is connected to an adjacent tab by at least one spring, and wherein an axial stiffness of said at least one spring is decreasing along the length of the device.
 7. The device according to claim 5, wherein each tab is connected to a pressure source for applying a pre-stressing force to the device using a spring.
 8. The device according to claim 5, wherein each one of said tabs is connected to an associated pressure source.
 9. The device according to claim 1, wherein the device is attached to the cover plate via an intermediate medium including at least one of a strip of fiber reinforced polymer, a wooden strip, and a plastic strip.
 10. A method for reinforcing a structural member by applying a pre-stress to a cover plate being attached to the structural member, comprising: at least temporarily securing a stress member to the cover plate using an attachment structure, and applying force to the stress member such that when the force is generated, different portions of the stress member will immediately be subjected to different magnitudes of force.
 11. A pre-stressing device for reinforcing or repairing a structural member by a pre-stressed cover plate, comprising: a stress member being connected to an attachment structure for at least temporarily securing said device to the cover plate, wherein the stress member has a varying axial stiffness along a longitudinal direction such that when a force is applied to the stress member, different portions of the cover plate will immediately be subjected to different magnitudes of stress.
 13. The device of claim 12, wherein the stress member further comprises a body having a tapered shape.
 14. The device of claim 12, wherein the stress member further comprises a series of tabs, each one of said tabs being connected to an associated pressure source such that when different forces are applied to the individual tabs, the different portions of the cover plate will immediately be subjected to different magnitudes of stress. 